There has been a recent increase in the number of reported cases of tuberculosis (TB), an old scourge and still the leading cause of death among infectious diseases. In addition, multidrug resistant strains of Mycobacterium tuberculosis, the bacterium that causes TB, have been isolated. Approximately one-third of the world's population harbors M. tuberculosis, and the World Heath Organization (WHO) predicts that by the year 2005, TB will kill 4 million people per year. In the process of screening for strain-specific antibodies, as well as developing subunit vaccines and efficient diagnostic tests, many murine monoclonal antibodies (MAbs) have been generated against various mycobacterial antigens. Using these MAbs and recombinant DNA techniques, genes of more than a dozen mycobacterial antigens have been cloned. Interestingly, several of these major antigens have sequence homologies to the conserved heat shock proteins. The 16 kDa protein carries epitopes restricted to tubercle bacilli (i.e., M. tuberculosis, M. africanum, and M. bovis) on the basis of B-cell recognition and is effective for diagnostic uses. This antigen, which is capable of generating cell-mediated immune responses, also contains T-cell epitopes which are cross-reactive with M. leprae antigens. It is the cell-mediated immune system which plays the dominant role in mycobacterial immunity. We used cryomicroscopy and image processing to reveal that this oligomeric antigen forms trimer of trimer.